The present invention relates to an index servo type magnetic head positioning control apparatus having a function for correcting thermal off-track in a magnetic disk apparatus, and a method thereof.
In magnetic disk (hard disk) apparatuses, the positional relationship between the magnetic head and a hard disk having a metal base is shifted due to heat, in other wards, a so-called "thermal off-track" phenomenon occurs. For this reason, when the magnetic head seeks a target track position, positional correction must be performed to absorb thermal off-track at the target track position.
In a conventional magnetic head positioning apparatus adopting an index servo method, an amount of thermal off-track is calculated based on servo data (SD) recorded on an index section of a data surface of the hard disk. A positional correction value (to be referred to as a thermal offset value hereinafter) of the magnetic head is determined in accordance with the calculated amount of thermal off-track, and a correction current corresponding to the thermal offset value is supplied to a stepping motor to drive a carriage, thereby executing positional correction of the magnetic head.
In the index servo method, the correction current is determined so that the peak values of servo data SD signals are equal to each other when servo data SD recorded on adjacent tracks are read out by the magnetic head. When the magnetic head is at a wrong position, since the peak values from the servo data differ, the magnetic head positioning operation is repeated, and a new correction current is set until the peak values from the servo data signals are equal to each other.
When the peak values from the servo data are monitored, it can be discriminated whether or not the magnetic head is at the center position of the data track. The amount of off-track of the magnetic head is calculated from the difference between the peak values (difference between voltage levels). The correction current corresponding to the calculated amount is supplied to the stepping motor, which serves as a drive source of the carriage, thereby correcting the magnetic head position.
In the conventional index servo method, since servo data SD is obtained once per rotation, the magnetic head must stand by until it detects the servo data. Therefore, when positional correction must be performed at respective track positions, positioning control takes a great deal of time, resulting in low throughput of the magnetic disk apparatus.
In order to solve the above problem, another method has been proposed wherein a correction value corresponding to an amount of thermal off-track at the target PG,4 track position is estimated, and magnetic head positioning control is performed based on a correction current corresponding to the estimated correction value. However, the thermal off-track values vary widely with changes in temperature, and it is impossible to accurately determine the thermal offset value for magnetic head positioning control. In addition, the relationship between the peak value difference for servo data SD and the amount of off-track must be constant at the inner and outer track positions of the disk or in individual apparatuses. For this purpose, a precise automatic gain control (AGC) circuit is required, leading to increased cost. In order to keep the relationship between the amount of off-track and the correction current constant in each apparatus, variations in wear of the carriage or in deviation of the stepping motor must be minimized. Therefore, since variations in characteristics of constituents of the magnetic disk apparatus must be minimized and an accurate AGC is required, total cost of the apparatus increases.